This invention relates to the solventless preparation of ester-substituted diaryl carbonates and in particular to a solventless method of making bis-methyl salicyl carbonate. In addition the present invention relates to a methods of preparing ester-substituted diaryl carbonates which minimize the use of organic solvents.
Ester-substituted diaryl carbonates such as bis-methyl salicyl carbonate have proven to be useful starting materials in the preparation of polycarbonates via the melt reaction of a diaryl carbonate with aromatic dihydroxy compounds. See for example, U.S. Pat. No. 4,323,668 in which rates of polymerization of bis-methyl salicyl carbonate with bisphenol A were shown to be higher than the corresponding rates of polymerization of bisphenol A with an unsubstituted diaryl carbonate, diphenyl carbonate. Notwithstanding the simplicity of its structure there are few reported preparations of ester-substituted diaryl carbonates.
A classical preparation of diaryl carbonates involves the reaction of a hydroxy aromatic compound such as phenol with phosgene gas in a two phase reaction system comprising water, an acid acceptor such as sodium hydroxide and a solvent such as methylene chloride or chloroform. Typical interfacial conditions used to prepare diphenyl carbonate (DPC) utilize water and methylene chloride phases, sodium hydroxide as a pH control measure and triethylamine as a catalyst. Under such conditions it is possible to convert phenol to DPC in essentially quantitative yield. Features of this process include the passage of phosgene gas into a reaction mixture comprising phenol and an organic solvent, and the removal of excess phosgene from the reaction mixture by passage of an inert gas such as nitrogen through the reaction mixture following completion of the phosgenation step. In both instances, solvent is entrained out of the reaction mixture by the flowing gases and must be trapped and recovered. Containment systems for preventing the escape of volatile organic compounds such as solvent emanating from reaction vessels frequently represents a significant cost of equipment used in chemical manufacturing.
Known methods for the preparation of diaryl carbonates such as diphenyl carbonate suffer in that their application to the preparation to ester-substituted diaryl carbonates results in only modest conversion of starting ester-substituted phenol to product ester-substituted diaryl carbonate and such known methods employ organic solvents during critical stages of the process where solvent containment is most difficult. The use of reaction systems which do not employ organic solvents to effect chemical transformations is desirable in that both environmental enhancements and reduction in manufacturing costs are made possible.
It would be desirable, therefore, to discover means for the efficient preparation of ester-substituted diaryl carbonates generally. In addition, it would be highly desirable to discover an efficient means of making ester-substituted diaryl carbonates from ester-substituted phenols and phosgene in a reaction system which avoids entirely or minimizes the use of organic solvents. Moreover, it would be desirable to use such a reaction system for the preparation of bis-methyl salicyl carbonate from methyl salicylate and phosgene.
The present invention provides a method of preparing ester-substituted diaryl carbonates, said method having a contact time, said method comprising contacting an ester-substituted phenol with phosgene and a catalyst and an aqueous phase in a mixture free of solvent, wherein the aqueous phase is maintained at a pH of at least about 8.3 throughout the contact time, said phosgene being used in an amount corresponding to between about 0.10 and about 1.20 moles of phosgene per mole of ester-substituted phenol.
The present invention further relates to a method of preparing bis-methyl salicyl carbonate, an ester substituted diaryl carbonate showing promise as a starting material for the synthesis of polycarbonates incorporating methyl salicyl endgroups.
The present invention may be understood more readily by reference to the following detailed description of preferred embodiments of the invention and the examples included therein. In the following specification and the claims which follow, reference will be made to a number of terms which shall be defined to have the following meanings:
The singular forms xe2x80x9caxe2x80x9d, xe2x80x9canxe2x80x9d and xe2x80x9cthexe2x80x9d include plural referents unless the context clearly dictates otherwise.
xe2x80x9cOptionalxe2x80x9d or xe2x80x9coptionallyxe2x80x9d means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not.
As used herein the term xe2x80x9cpolycarbonatexe2x80x9d refers to polycarbonates incorporating structural units derived from one or more dihydroxy aromatic compounds and includes copolycarbonates and polyester carbonates.
As used herein, the term xe2x80x9cmelt polycarbonatexe2x80x9d refers to a polycarbonate made by a process comprising the transesterification of a diaryl carbonate with a bisphenol.
xe2x80x9cCatalytically effective amountxe2x80x9d refers to the amount of the catalyst at which catalytic performance is exhibited.
As used herein the term xe2x80x9ccontact timexe2x80x9d is used interchangeably with reaction time.
As used herein the term xe2x80x9csolvent freexe2x80x9d is used interchangeably with the terms xe2x80x9cfree of solventxe2x80x9d and xe2x80x9csolventlessxe2x80x9d.
As used herein the term xe2x80x9calkyl radicalxe2x80x9d refers to a radical having a valence of at least one comprising a linear or branched array of atoms which is not cyclic. The array may include heteroatoms such as nitrogen, sulfur and oxygen or may be composed exclusively of carbon and hydrogen. Examples of alkyl radicals include methyl, methylene, ethyl, ethylene, hexyl, hexamethylene and the like.
As used herein the term xe2x80x9caromatic radicalxe2x80x9d refers to a radical having a valence of at least one comprising at least one aromatic group. Examples of aromatic radicals include phenyl, pyridyl, furanyl, thienyl, naphthyl, phenylene, and biphenyl. The term includes groups containing both aromatic and aliphatic components, for example a benzyl group.
As used herein the term xe2x80x9ccycloalkyl radicalxe2x80x9d refers to a radical having a valance of at least one comprising an array of atoms which is cyclic but which is not aromatic. The array may include heteroatoms such as nitrogen, sulfur and oxygen or may be composed exclusively of carbon and hydrogen. Examples of cycloalkyl radicals include cyclcopropyl, cyclopentyl cyclohexyl, tetrahydrofuranyl and the like.
It has been discovered that ester-substituted phenols such as methyl salicylate are efficiently converted to ester-substituted diaryl carbonates such as bis-methyl salicyl carbonate under mild reaction conditions in a reaction system which is free of solvent. The present invention provides a method of converting an ester-substituted phenol to an ester-substituted diaryl carbonate, said method comprising contacting in a reaction mixture the ester-substituted phenol, a catalyst, an aqueous phase, and phosgene under conditions which promote the reaction of the ester-substituted phenol with the phosgene in the absence of a solvent other than water or the ester-substituted phenol itself.
The method of the present invention provides a means for the preparation of ester-substituted diaryl carbonates in a reaction system which is free of solvent. By xe2x80x9cfree of solventxe2x80x9d it is meant that no solvent other than water or starting ester-substituted phenol is present in the reaction mixture during the reaction of ester-substituted phenol with phosgene to afford intermediate ester substituted phenyl chloroformate and product ester-substituted diaryl carbonate. For example, in embodiments of the present invention wherein phosgene is contacted with a mixture comprising an aqueous phase, a catalyst and an ester-substituted phenol, no solvent other than water or the ester-substituted phenol itself is present during the conversion of the phosgene to intermediate ester-substituted phenyl chloroformate and product ester-substituted diaryl carbonate. Typically, the phosgene is contacted with the mixture comprising an aqueous phase, a catalyst and an ester-substituted phenol by addition of the phosgene to said mixture, for example by a step comprising addition of the phosgene gas to the mixture. In embodiments of the present invention wherein excess or residual phosgene remains following reaction with the ester-substituted phenol, the product mixture may be purged with an inert gas to remove the excess or residual phosgene. According to the method of the present invention the product mixture contains no solvent other than water or ester-substituted phenol while the product mixture is being purged with an inert gas to remove excess or residual phosgene. The method of the present invention, by eliminating the presence of solvents other than water or the ester-substituted phenol itself during phosgene addition and phosgene removal steps, serves to reduce the need for the solvent containment measures which would be required if an organic solvent such as methylene chloride were present during these steps.
In instances where traces of ester-substituted phenyl chloroformates are persistent, a solvent such as methylene chloride may be added according to the method of the present invention in order to aid the complete conversion of intermediate ester-substituted phenyl chloroformates to product ester-substituted diaryl carbonates. Additionally, one or more solvents may be used during the isolation of the product.
In one aspect, the present invention provides a method for the efficient preparation of an ester-substituted diaryl carbonate having structure I 
wherein R1 is independently at each occurrence C1-C20 alkyl radical, C4-C20 cycloalkyl radical or C4-C20 aromatic radical, R2 is independently at each occurrence a halogen atom, cyano group, nitro group, C1-C20 alkyl radical, C4-C20 cycloalkyl radical, C4-C20 aromatic radical, C1-C20 alkoxy radical, C4-C20 cycloalkoxy radical, C4-C20 aryloxy radical, C1-C20 alkylthio radical, C4-C20 cycloalkylthio radical, C4-C20 arylthio radical, C1-C20 alkylsulfinyl radical, C4-C20 cycloalkylsulfinyl radical, C4-C20 arylsulfinyl radical, C1-C20 alkylsulfonyl radical, C4-C20 cycloalkylsulfonyl radical, C4-C20 arylsulfonyl radical, C1-C20 alkoxycarbonyl radical, C4-C20 cycloalkoxycarbonyl radical, C4-C20 aryloxycarbonyl radical, C2-C60 alkylamino radical, C6-C60 cycloalkylamino radical, C5-C60 arylamino radical, C1-C40 alkylaminocarbonyl radical, C4-C40 cycloalkylaminocarbonyl radical, C4-C40 arylaminocarbonyl radical, and C1-C20 acylamino radical; and b is independently at each occurrence an integer 0-4.
Examples of ester-substituted diaryl carbonates which may be prepared using the method of the present invention include bis-methyl salicyl carbonate (CAS Registry No. 82091-12-1), bis-ethyl salicyl carbonate, bis-propyl salicyl carbonate, bis-butyl salicyl carbonate, bis-benzyl salicyl carbonate, bis-methyl 4-chlorosalicyl carbonate and the like. Typically bis-methyl salicyl carbonate is preferred for use in melt polycarbonate synthesis due to its lower molecular weight and higher vapor pressure.
The ester-substituted phenol starting materials according to the present invention include at least one phenol having structure II 
wherein R1 and R2 are defined as in structure I, and b is an integer 0-4.
Examples of ester-substituted phenols which may serve as starting materials for the method of the present invention include methyl salicylate, ethyl salicylate, isopropyl salicylate, propyl salicylate, butyl salicylate, benzyl salicylate, methyl 4-chlorosalicylate and the like.
The method of the present invention relies upon at least one catalyst to accelerate the reactions of phosgene and intermediate chloroformates with ester substituted phenols. Suitable catalysts include phase transfer catalysts and tertiary amine catalysts.
Suitable phase transfer catalysts are widely available and include quaternary ammonium salts of aliphatic amines, quaternary ammonium salts of aromatic amines, quaternary phosphonium salts, sulfonium salts, polyethers and the like. Quaternary ammonium salts of aliphatic amines are illustrated by methyl tributyl ammonium chloride, tetramethyl ammonium chloride and the like. Quaternary ammonium salts of aromatic amines are illustrated by N-benzyl pyridinium chloride, N-benzyl 4-Nxe2x80x2, Nxe2x80x2-dimethylamino pyridinium chloride and the like. Quaternary ammonium salts include hexaalkyl guanidinium compounds such as hexaethyl guanidinium chloride. Quaternary phosphonium salts are illustrated by tetrabutyl phosphonium acetate and the like. Sulfonium salts are illustrated by trimethyl sulfonium chloride and the like. Polyethers are illustrated by polyethylene glycol and crown ethers such as 18-crown-6 and the like.
In one embodiment of the present invention the phase transfer catalyst is at least one quaternary ammonium compound having structure III 
wherein R3-R6 are independently a C1-C20 alkyl radical, C4-C20 cycloalkyl radical or a C4-C20 aryl radical and Xxe2x88x92 is at least one organic or inorganic anion. Suitable anions Xxe2x88x92 include hydroxide, halide, carboxylate, sulfonate, sulfate, carbonate and bicarbonate.
Where Xxe2x88x92 is a polyvalent anion such as carbonate or sulfate it is understood that the positive and negative charges in structure III are properly balanced. For example, where R3xe2x88x92R6 in structure III are each methyl groups and Xxe2x88x92 is carbonate, it is understood that Xxe2x88x92 represents xc2xd (CO3xe2x88x922).
Quaternary ammonium compounds having structure III which are suitable for use as phase transfer catalysts according to the method of the present invention are illustrated by methyl tributyl ammonium chloride, tetrabutyl ammonium chloride, and decyl trimethyl ammonium chloride.
The amount of phase transfer catalyst employed is in a range between about 0.1 and about 2 mole percent, preferably between about 0.25 and about 1.0 mole percent catalyst per mole of ester-substituted phenol employed.
In one embodiment of the present invention a tertiary amine is also included as a co-catalyst for the formation of ester-substituted diaryl carbonates. The tertiary amine has been found to accelerate the formation of ester-substituted diaryl carbonate product and to act to minimize the presence of the intermediate ester-substituted phenyl chloroformate in the product. The introduction of a tertiary amine into the reaction mixture after phosgene addition has been completed has been found useful in reaction systems in which the chloroformate intermediates tend to persist. Thus, phosgene addition to reaction mixtures comprising an aqueous phase, an acid acceptor, an ester-substituted phenol and a phase transfer catalyst according to the method of the present invention may at times result in a product mixture comprising ester-substituted diaryl carbonate and the intermediate ester-substituted phenyl chloroformate. Typically, the amount of ester-substituted phenyl chloroformate is low, less than 1 mole percent based upon the total number of moles of ester-substituted phenol employed, but its presence in the product is undesirable. It has been found that a small amount of a tertiary amine added following the phosgenation step provides a means of eliminating residual chloroformates present in the product mixture. Typically, the amount of tertiary amine co-catalyst used is in a range between about 0.01 mole and about 1 mole percent based upon the total number of moles of ester-substituted phenol employed.
In one embodiment of the present invention no phase transfer catalyst is employed and at least one tertiary amine is used as the catalyst. In this embodiment of the present invention the tertiary amine may be added prior to, concurrently with, or following phosgene addition. Typically, the amount of tertiary amine catalyst used is in a range between about 0. 1 mole and about 2 mole percent based upon the total number of moles of ester-substituted phenol employed.
Tertiary amines suitable for use as catalysts or co-catalysts according to the method of the present invention are illustrated by triethylamine, diispropyl ethyl amine, tributylamine, and 1,4-diazabicyclooctane.
According to the method of the present invention the aqueous phase is maintained at a pH above about 8.3 throughout the reaction. In one embodiment of the present the present invention the pH of the aqueous phase is maintained in a range between about 8.3 and about 12, preferably between about 8.3 and about 10.3 and still more preferably between about 9.3 and about 10.3. The pH of the aqueous phase may be controlled by the addition of an aqueous solution of an inorganic base such as a metal hydroxide. Suitable metal hydroxides include alkali metal hydroxides such as sodium hydroxide and lithium hydroxides, and alkaline earth metal hydroxides such as calcium hydroxide and magnesium hydroxide. Sodium hydroxide is preferred.
The method of the present invention provides a means for the reaction of ester-substituted phenols with phosgene and intermediate chloroformates at reaction rates which are sufficiently rapid to allow the efficient preparation of ester-substituted diaryl carbonates. Thus, the method of the present invention requires a relatively short period of contact between the reactants, usually on the order of minutes. In embodiments of the present invention wherein the amount of phosgene employed is not in a molar excess over the amount of ester-substituted phenol employed, substantially all of the phosgene is converted to product ester-substituted diaryl carbonate and residual ester-substituted phenyl chloroformate after a period of contact between the reactants of between about 5 and about 60 minutes. The expression xe2x80x9csubstantially all of the phosgenexe2x80x9d means at least about 95 percent of the phosgene introduced into the reaction mixture.
The period of contact between the reactants is referred to as the contact time or the reaction time. Contact times are in a range between about 5 and about 60 minutes, preferably between about 5 and about 45 minutes, and still more preferably between about 5 and about 30 minutes. Contact time is measured from the point of first contact between all of the reactants; phosgene, the ester substituted phenol, the catalyst and the aqueous phase having a pH of above about 8.3, until no further reaction is observed by HPLC or like analytical technique.
The temperature of the reaction mixture is typically maintained between about 0xc2x0 C. and about 100xc2x0 C., preferably between about 0xc2x0 C. and about 80xc2x0 C., and still more preferably between about 0xc2x0 C. and about 50xc2x0 C. In some embodiments of the present invention a temperature of the reaction mixture between about 5xc2x0 C. and about 15xc2x0 C. is preferred. The temperature of the reaction mixture may be varied during the contact time. For example, the reaction mixture may be controlled at temperature of between about 5xc2x0 C. and about 15xc2x0 C. during a phosgene addition step and at ambient temperature just prior to workup as the last traces of chloroformate intermediate are converted to ester-substituted diaryl carbonate.
The ratio of phosgene to ester substituted phenol may be varied according to the method of the present invention between about 0.1 to about 1.2 moles of phosgene per mole of ester-substituted phenol employed. Generally it is preferred to use an excess of the ester-substituted phenol relative to the amount of phosgene employed. Thus it is preferable that the ratio of phosgene to ester-substituted phenol be in a range between about 0.1 and about 0.95 moles phosgene per mole ester-substituted phenol, preferably between about 0.2 and about 0.8 moles phosgene per mole ester-substituted phenol.
The method of the present invention may be carried out in either a batch mode or a continuous fashion. In embodiments of the present invention operated in a batch mode the reactants are contacted in a reaction vessel such as a stirred tank reactor. Typically, the product ester-substituted diaryl carbonate precipitates from the reaction mixture as a solid which may recovered by filtration and like techniques or by centrifugation and like techniques. In embodiments of the method of the present invention is operated in a continuous mode, the reactants are introduced in a continuous fashion into a reactor, such as a tubular reactor, adapted for continuous introduction of reactants and continuous removal of a product stream. Solid product diaryl carbonate may recovered from the product stream by filtration as on a rotary filtration system such as a Bird filter.
In some instances of batch mode or continuous mode embodiments of the present invention, the product ester-substituted diaryl carbonate may not be recovered as a solid and may be conveniently isolated as a liquid. This is particularly so in cases where the product ester-substituted diaryl carbonate at its state of purity in the product mixture has a low melting point, for example less than 100xc2x0 C. In instances where excess ester-substituted phenol is employed the product may be isolated as a liquid wherein the excess ester-substituted phenol in the product mixture acts as a solvent.